There are various conventional methods of refining sucrose (beet sugar) abstracted from sugar beet, representative examples of which include the following methods having respective merits and demerits.
A method (1) is one comprising sugar beet cutting, extraction, carbonation (coagulation and sedimentation for removal of impurities through adsorption thereof on a precipitate of calcium carbonate during formation of the precipitate by adding lime milk to raw juice obtained by extraction and blowing carbon dioxide therein), filtration, softening (removal of hardness components such as Ca and Mg with cation exchange resin in Na form), concentration, and boiling (boiling sucrose crystals out of the concentrate through crystallization). This method is simple because no demineralization is done although softening is done, but involves demerits of poor sucrose crystallization during the boiling step and hence formation of a large amount of molasses because of an insufficient sucrose purity of the sucrose-containing concentrate as the object of boiling.
A method (2) is one comprising the same sugar beet cutting, extraction, carbonation and filtration as in the method (1), and further comprising subsequent softening and demineralization by ion exchange treatment (removal not only of hardness components such as Ca and Mg but also of other salt components), concentration, and boiling. Use of the following four kinds of ion exchange resins: a strongly acidic cation exchange resin, a weakly basic anion exchange resin, a strongly basic anion exchange resin and a weakly acidic cation exchange resin in this order is best for the ion exchange treatment in this method. This method does not involve the demerits of the method (1), but involves demerits of frequent regeneration of the ion exchange resins, use of large amounts of regenerants, and various troublesome treatments of regeneration waste because much salts are contained in sucrose-containing filtrate as the object of ion exchange treatment to decrease the throughput per unit quantity of the ion exchange resins. In view of much salts as mentioned above, decomposition of sucrose by the cation exchange resin in the hydrogen ion form (H form) in the first stage of ion exchange treatment must be avoided by cooling the above-mentioned sucrose-containing filtrate once to at most 10.degree. C. for the treatment thereof with the cation exchange resin in the H form (so-called cold process demineralization), and the resultant treated solution must be heated up to a temperature of 50 to 55.degree. C. for the second stage of ion exchange treatment thereof (polishing for demineralization, decoloration, deodorization, etc.), thus giving rise to demerits of complexity of operations and large energy costs.
A method (3) is a recently proposed one comprising filtration, softening, concentration, demineralization by ion exclusion chromatographic separation, concentration, and boiling without carbonation after the same extraction as in the foregoing 2 methods (PCT International Publication No. WO 95/16794). This method, which does not involve the demerits of the method (2) but indispensably requires filtration in order to avoid clogging of a chromatographic separator and an increase in the pressure loss therethrough (no details of filtration are described in the above-mentioned patent literature), involves demerits such as an incapability of removal of sticky substances derived from the plant (beet) and called "gum" as well as colloidal substances, a difficulty in filtration, a great cost involved in the filtration step, so grave a pressure loss in chromatographic separation due to colloidal substances unremovable by filtration as to result in a failure in liquid passage through the chromatographic separator in an extreme case, an unavoidable decrease in the feed rate of a starting solution (starting chromatographic solution) as an object of chromatographic separation for decreasing the pressure loss in the chromatographic separation operation, and a failure in obtaining such high-quality sucrose crystals as in the method (2) due to insufficient demineralization. The cause of all such demerits is that the coagulation and sedimentation step such as the carbonation step is not taken.
Demineralization of a beet sugar solution before boiling according to such ion exchange treatment or chromatographic separation is aimed at improving the quality of sucrose crystals precipitated by later boiling. Further, sucrose is recovered as much as possible by repeating demineralization (by ion exchange or chromatographic separation), concentration, and boiling of molasses obtained after boiling.
Ion exchange resins used in demineralization by such conventional ion exchange as in the method (2) must be subjected to the regeneration step at a certain point of time. Regenerant chemicals for use in this step and washing water for use in the subsequent washing step pose a problem of raising the cost of demineralization. Further, various treatments of regeneration waste discharged in the regeneration and washing steps are so troublesome as to raise the product cost. On the other hand, when demineralization is done by chromatographic separation like in the method (3), the throughput is limited if a high separability of components is to be attained, with the result that the separator must inevitably be scaled up to pose a problem of a high construction cost thereof. In chromatographic separation, sucrose as the desired component is diluted with eluent water to pose another problem of raising the running cost of the later concentration step. Still another problem is a loss of the desired component (sucrose) involved in separation.
An object of the present invention, which has been made in view of the foregoing problems of the prior art technologies, is to provide a process for efficiently demineralizing a beet sugar solution before boiling.